Group III-V compounds are important and widely used semiconductor materials. Group III nitrides in particular have wide, direct band gaps, which make them particularly useful for fabricating optic components (particularly, short wavelength LEDs and lasers) and certain electronic components (particularly, high-temperature/high-power transistors).
The Group III nitrides have been known for decades to have particularly advantageous semiconductor properties. However, their commercial use has been substantially hindered by the lack of readily available single crystal substrates. It is a practical impossibility to grow bulk single crystal substrates of the Group III-nitride compounds using traditional methods, such as Czochralski, vertical gradient freeze, Bridgeman or float zone, that have been used for other semiconductors such as silicon or GaAs. The reason for this is the high binding energy of the Ga—N bond which results in decomposition, and not melting of GaN at atmospheric pressure. Very high pressure and temperatures (2500° C. and >4 GPa pressure are required to achieve melted GaN. While various high pressure techniques have been investigated, they are extremely complicated and have lead to only very small irregular crystals. (A. Denis et al, Mat. Sci. Eng. R50 (2006) 167.)
The lack of a native single crystal substrate greatly increases the difficulty in making epitaxial Group III-nitride layers with low defect densities and desirable electrical and optical properties. A further difficulty has been the inability to make p-type GaN with sufficient conductivity for use in practical devices. Although attempts to produce semiconductor grade GaN began at least in the early 1970s, no usable progress was made until the late 1990's when two breakthroughs were developed. The first was the use of low temperature GaN and AlN buffer layers which led to acceptable growth of Group III-nitride layers on sapphire. The second was the development of a process to achieve acceptable p-type conductivity. In spite of these technological advances, the defect density in Group III-nitride layers is still extremely high (1E9-1E11 cm−3 for dislocations) and the p-type conductivity is not as high as in other semiconductors. Despite these limitations, these advances led to commercial production of III-nitride epitaxial films suitable for LEDs (see, e.g., Nakamura et al, 2nd ed. 2000, The Blue Laser Diode, Springer-Verlag, Berlin).
The high defect density is a result of growth on a non-native substrate. Sapphire is the most widely used substrate, followed by silicon carbide. Differences in the lattice constant, thermal coefficient of expansion and crystal structure between the III-nitride epitaxial layer and the substrate lead to a high density of defects, stress and cracking of the III-nitride films or the substrate. Furthermore, sapphire has a very high resistivity (cannot be made conductive) and has poor thermal conductivity.
SiC substrates can be produced in both conductive and highly resistive forms, but is much more expensive than sapphire and only available in smaller diameters (typically 50 mm diameter with 150 mm and 200 mm as demonstrations). This is in contrast to sapphire and native substrates for other semiconductors such as GaAs and silicon, which are available at lower cost and in much larger diameters (150 mm diameter for sapphire; 300 mm for GaAs).
While the use of sapphire and SiC are suitable for some device applications, the high defect density associated with III-nitride layers grown on these substrates leads to short lifetime in laser diodes. III-nitride laser diodes are of particular interest because their shorter wavelength permits much higher information density in optical recording methods. It is expected that substrates with lower defect densities will lead to higher brightness LEDs which are required for replacement of incandescent and fluorescent bulbs. Finally, Group III-nitride materials have desirable properties for high frequency, high power electronic devices but commercialization of these devices has not occurred, in part because of substrate limitations. The high defect density leads to poor performance and reliability issues in electronic devices. The low conductivity of sapphire makes it unsuitable for use with high power devices where it is vital to be able to remove heat from the active device region. The small diameter and high cost of SiC substrates are not commercially usable in the electronic device market, where larger device sizes (compared to lasers or LEDs) require lower cost, large area substrates.
A large number of methods have been investigated to further reduce the defect density in epitaxial III-nitrides on non-native substrates. Unfortunately the successful methods are also cumbersome and expensive and non-ideal even if cost is not an object. One common approach is to use a form of epitaxial lateral overgrowth (ELO). In this technique the substrate is partially masked and the III-nitride layer is coerced to grow laterally over the mask. The epitaxial film over the mask has a greatly reduced dislocation density. However, the epitaxial film in the open regions still has the same high dislocation density as achieved on a non-masked substrate. In addition, further defects are generated where adjacent laterally overgrown regions meet. To further reduce the dislocation density, one can perform multiple ELO steps. It is clear that this is a very expensive and time consuming process, and in the end produces a non-homogeneous substrate, with some areas of low dislocation density and some areas with high dislocation density.
The most successful approach to date to reducing defect densities is to grow very thick layers of the III-nitride material. Because the dislocations are not oriented perfectly parallel with the growth direction, as growth proceeds, some of the dislocations meet and annihilate each other. For this to be effective one needs to grow layers on the order of 300 to 1000 μm. The advantage of this approach is that the layer is homogeneous across the substrate. The difficulty is finding a growth chemistry and associated equipment that can practically achieve these layer thicknesses. MOVPE or MBE techniques have growth rates on the order of less than 1 to about 5 μm/hour and thus are too slow, even for many of the ELO techniques discussed above, which require several to tens of microns of growth. The only growth technique that has successfully achieved high growth rates is hydride vapor phase epitaxy (HVPE).
In summary, the current state of the art in producing low dislocation Group III nitride material is to use HVPE to produce very thick layers. However the current HVPE process and equipment technology, while able to achieve high growth rates, has a number of disadvantages. The present invention now overcomes these disadvantages and provides relatively low cost, high quality Group II nitrides which lead to new, innovative applications, e.g., in residential and commercial lighting systems.